Electric wave filter



May 2, 1939.

w. s. PERCIVAL ELECTRIC WAVE FILTER Filed Feb. 18, 1957 M/VENTUR WILL/AM PERC/VAL y Ami L,

A TTORA/EY Patented May 2, 1939 UNITED STATES PATENT OFFICE ELECTRIC WAVE FILTER Application February 18, 1937, Serial No. 126,317 In Great Britain February 21, 1936 6 Claims.

This invention relates to electric wave filters, and is particularly but not exclusively concerned with wave filters for use in thermionic amplifiers which are required to have a considerable stage gain while nevertheless providing a substantially uniform amplification within a wide range of frequency.

In an amplifier of a known kind illustrated in Fig. 1 of the accompanying drawing, provided 1 that the anode load impedance R is small compared to the impedance of valve V1, then it can be shown that where G is the stage gain provided by the amplifier stage including valve V1, f is the pass band of the amplifier in cycles per second, 9 is the mutual conductance of valve V1, C is the total capacity which is efiectively in shunt with the anode impedance R and k is a constant; the coupling between valves V1 and V2 acts as a low pass filter, and f the cut-off frequency is determined by 21rfCR=L Hence for a simple resistance coupled amplifier.

The same value for k is obtained if the coupling is converted, by shunting grid leak Hg with an inductance, into a band pass filter of which the band width is small compared with the midfrequency of the pass band.

Now the magnitude of constant 70 determines the magnitude of the product G and if f is 5 large, as for example in an amplifier for television purposes, a large stage gain (G) can only be obtained by arranging that 70 has a substantial value, since the effective shunt capacity (0) cannot be reduced below a minimum value de- 40 termined by the valve employed. It is known that the value of R: can be increased in an arrangement such as is shown in Fig. 1 by connecting an inductance of suitable magnitude either in series with resistance R or in the lead from the anode of valve V1 to the control grid of valve V2.

It is found, however, that a considerably greater increase in the value of k can be obtained by coupling the two valves in known manner by means of a low-pass or band-pass filter which is terminated by an impedance which approximates to the characteristic impedance of the filter. The value of 7c is still further increased by arranging in known manner that the 6,5- lnterelectrode capacities of the valves constitute the whole or a part of the shunt capacities of the filter.

It will be appreciated from the above that in amplifiers where it is desirable to obtain a considerable stage gain times band-pass it is desirable to employ a filter network for coupling purposes.

A filter is usually built up of a number of prototype half sections, and the manner in which these sections are joined determines whether the filter is composed of 1r-type or T-sections. In addition, the arrangement of the inductances or condensers either in series or in shunt determines the type of filter, that is to say, low-pass or highpass filter, and if the condensers are arranged in shunt and are tuned with inductances to a particular frequency, and the sections so formed are suitably coupled, a band-pass filter is obtained. Variations of these types of filter are also known, and in particular a filter known as a band-stop filter is suitable for use in conjunction with the present invention. Since these filters are composed of half sections, the initial reactance at the end remote from the usual termination is identical to the corresponding shunt reactance of an elementary half section. It is found however with filters of the known kind that the impedance is not constant over the pass-band. Hence in cases where it is desired to employ a filter to couple a thermionic valve with a further valve or equivalent device to provide substantially uniform amplification over a wide range of frequency, filters of the known kind are not altogether suitable owing to the variation of the input impedance.

It is the chief object of the present invention to provide an improved wave filter, particularly for use as an inter-valve coupling which will afford a substantially constant input impedance over the pass-band for which the filter is designed.

According to the invention an electric wave filter is provided comprising one or more identical prototype half-sections or half-sections derived from such prototype half-sections, each halfsection comprising a series and a shunt reactance, one end of the filter being terminated by an impedance substantially equal to the characteristic impedance of the filter, in which for the purpose of maintaining the impedance at the unterminated end of the filter substantially constant over the pass-band, the initial series or shunt reactance as the case may be, is made equal in value to a full series or shunt reactance.

Thus, in the case of a filter of the low-pass type comprising series reactances and shunt condensers, the condenser at the unterminated end is made equal in value to the full shunt value. This distinguishes from the known types of filter since in an equivalent low-pass filter the capacity at the unterminated end will be half the value of a full shunt element, that is to say, it will be equal to the shunt capacity of an elementary half-section. The invention can be applied not only to filters of the low-pass type but to filters of the other kinds hereinbefore referred to.

Where the invention is applied for the purpose of coupling one thermionic valve to, for example, another thermionic valve, it is found that a considerably greater increase in the value of k can be obtained as stated above by employing the inter-electrode capacities of the valves as the whole or a part of the shunt capacities of the filter.

The termination may comprise a simple resistance of the appropriate value or is of the m-derived type as hereinafter referred to.

In the preferred form of the invention, the filter is of the low-pass type and comprises one and a half sections of a 1rtype network, each half-section of the network comprising a series inductance of value Lo and a shunt capacity of value Co; one end of the filter is terminated by the corresponding series derived m-type halfsection and a resistance, while the other end is shunted by a capacity of value 200, that is to say, twice the normal end capacity of a 1rtype filter. The filter is found to have an input impedance which is approximately constant almost up to the cut-off frequency; this can be demonstrated as follows:

The input impedance Z0 of the normal filter referred to above, assuming a perfect non-reflecting termination at the far end, is given by ZYWFEEYY where R is the nominal impedance of the filter and is given by a Rt 8., and

and represents the pulsatance at the cut-off frequency.

When the shunt capacity at the unterminated end is doubled, the input impedance Z of the modified network is given by Modulus [Z] termination, at all frequencies up to cut-01f frequency; the failure of the termination to match the characteristic impedance of the network will determine the departure from this condition which is experienced in the neighbourhood of the cut-01f frequency. 7

The same constant input impedance may be shown to result for all constant-K filters. Thus where the invention is applied to filters composed of one or more T sections the input impedance can be made constant if the series element at the end remote. from the termination corresponds in value to a full series element rather than the mid-series element as is usually the case.

In some cases instead of employing a series m-derived termination, or as will appear hereinafter a series mm'-derived termination, a shunt derived termination can be employed, but such form of termination provides a lower cut-oiT for a given total capacity.

Where the filter comprises a single half-section, the shunt or series reactance as the case may be, at the end remote from the termination will be made in accordance with the invention, the same value as the theoretical value of a full shunt or series reactance. One or more of the elementary prototype sections of the filter may be replaced in known manner by corresponding derived sections.

In order that the said invention may be clearly understood and readily carried into effect, the same will now be more fully described with reference to the accompanying drawing in which:

Figure 1 has already been referred to and in which Figures 2 and 3 illustrate two embodiments in accordance with the invention.

Figure 2 of the drawing illustrates a wave filter in accordance with the invention employed as an inter-valve coupling for an amplifier. As shown, the amplifier comprises two screen grid valves V1 and V2, the anode of the first being connected to the control grid of the second through a blocking condenser C1 and an inductance L1 in series. A grid leak Ry is provided for the second valve. The anode-to-cathode capacity of valve V1, together with its associated stray wiring and like capacities form a ir-EYZZJS low-pass filter of one and a half sections with inductance L1, part of inductance L2 and the control grid-to-cathode capacity of valve V2 and its associated stray capacities; the two capacities of the network are indicated by dotted lines.

The filter is terminated by the corresponding series derived m-type network, the termination comprising a part of inductance L2 connected in series with the anode load resistance R of valve V1 between the anode of valve V1 and the positive terminal of the associated anode current source (not shown), and a condenser 02 and an inductance L3 connected in series between the junction of R and L2, and the cathode; the remaining part of inductance L2 constitutes the series reactance of the first half-section of the .filter.

If inductance L1 has the value 2L0 and the effective shunt capacities of the networks the value 2C0, then the values of the three elements of the derived m-type termination are given by the relationships.

A suitable value for min the arrangement described is 0.7, and in this case the network presents a substantially constant impedance to valve V1 up to the cut-off frequency. When the two shunt capacities of the filter are made equal, the constant It has the value or greater. If the amplifier described forms a part of a multi-stage amplifier, the band width of which is defined as that corresponding to a reduction in amplitude of 5% in each stage,-the effective value of k: is further increased.

The values of the two shunt capacities may be made equal to one another in practice by adjusting the positions of the various connecting wires, and of the inductance L1, but preferably the capacities are equalised by the positioning of condenser C1 and leak resistance Rg either on the left or right hand side of inductance L1 so that their capacities to earth add either to the anode or grid capacities of valves V1 or V2 whichever is the smaller. I

It is found that the response of the arrangement to transients is improved by shunting L1 or any other reactance of the filter with a resistance the value of which may be between 10 and 20 times the value of resistance R.

It will be understood that the output shunt capacity of the network may be provided by a device other than the valve V2; in an example, the modulator-to-cathode capacity of a cathode ray tube constitutes the desired capacity.

The velocity of propagation through the coupling network may be made more nearly constant over the pass band by the provision of any known or suitable phase compensating means, which may take the form of a further derived m-type filter section connected between the anode of valve V1 and the control grid of valve Va. A suitable value for m in this case is 1.5.

The uniformity of the amplitude characteristic of the network may be improved by employing a termination of the derived mm-type, in which case a substantially flat characteristic can be obtained up to more than of the cut-off frequency.

When a triode valve is employed as valve V1, the anode impedance of the triode as a shunt across its anode-cathode capacity should be taken into account, and the values of elements C2, L2 and L3 will require to be reduced; the values of L1 and the resistance by which it is shunted, if one is employed, will, however, need only slight modification.

In some cases the grid-cathode capacity of the valve V2 may be somewhat larger than the anodecathode capacity of the first valve, and in a modification of the invention instead of increasing the smaller capacity in accordance with the main feature of the invention, it is left at the smaller value and the termination is associated with this smaller capacity as shown in Fig. 2. A better phase characteristic is thereby obtained. If the grid-cathode capacity of valve V1 is in practice smaller than the anode-cathode capacity of valve V1 and it is desired to effect the improvement in the phase characteristic, the termination may be associated with the grid-cathode capacity of valve V2, that is a reversal of the valve positions which is permissible by the principle of reciprocity. The inequality of capacities for obtaining a better phase characteristic may be accentuated by adjusting the positions of the various connecting wires and other elements as stated above.

Figure 3 of the drawing illustrates an arrangement according to the invention employing a band-pass filter as an inter-valve coupling. The elements in this figure which correspond to the equivalent elements in Fig. 2 bear the same reference characters. The band-pass filter in this case comprises the anode-cathode capacity of valve V1 and the inductance L5, the grid-cathode capacity of valve V2 and an inductance Le which are coupled together through an inductance L1 and a coupling condenser C1, the inductances L5, L6 and L1 tuning with their associated capacities to the mid-frequency of the pass-band. The filter is terminated by a termination of the series derived m-type which together with a series element of the filter is enclosed by the dotted line T, the termination being suitably modified compared with the termination shown in Fig. 2. The inductance L4 is a choke, which together with bypass condensers C10 play no part in the operation of the filter. In this example of the invention the reactance of the inductance L6 is made equal to the reactance of inductance L5 and the gridcathode capacity of valve Vg equal to the anodecathode capacity of valve V1.

It is to be understood that the values of the components of the filter are not critical and that for practical purposes variation of the values may be made and give satisfactory results if within about 30% of the theoretical values for optimum results. Consequently the appended claims are intended to cover cases in which variation of the values of the components is made within about 30%.

I claim:

1. An electric wave filter network comprising one or more identical prototype half-sections, each half-section comprising a series and a shunt reactance, and an impedance substantially equal to the characteristic impedance of the network terminating one end of said network, the shunt reactance at the end remote from the said termination being of a value equal to a full shunt reactance'of the filter network.

2. An electric wave filter as in claim 1, wherein the termination comprises a shunt m-derived half-section.

3. An electric wave filter as in claim 1, wherein the termination comprises a shunt mm'-derived half-section.

4. An electric wave filter comprising one or more half-sections of the 1r-type connected together to form a low pass network, each half-section comprising a series and a shunt reactance, a terminating impedance corresponding substantially to the characteristic impedance of the network at one end thereof, the shunt reactance at the end remote from the said termination being equal in value to a full shunt reactance of the filter network.

5. A low-pass filter having an input impedance which is approximately constant almost up to the cut-off frequency comprising one and one half sections of a Ir-type net-work, each half section comprising series inductance of value Lo and a shunt capacity of value Co, one end of the filter being terminated by the corresponding series derived m-type half-section and a resistance, the other end of the filter being shunted by a capacity of value 200.

6. In an amplifying circuit utilizing a pair of vacuum tubes in cascade, a resistance and an inductance serially connected between the positive terminal of a power supply source and the anode of the first tube, a circuit including a condenser and an inductance serially connected between the anode side of the resistance and cathode, and an inductance connected between theanode of the first tube and the control grid of the second; the

latter inductance forming with the anode to the cathode capacity of the first tube and the control grid to cathode capacity of the second tube a low-pass filter of one and one half sections, the series condenser and inductance together with a part of the inductance connected in series with the anode resistance forming the terminating impedance of the filter, and the remaining part of the series anode inductance constituting the series reactance of the first half section of the filter.

WILLIAM SPENCER PERCIVAL. 

